Atypical neuroleptics such as risperidone and quetiapine are being used frequently in children and adolescents with severe psychopathology. The effects of these agents on normal growth and development are currently unknown. This project will treat 48 normally developing young pigtail monkeys (Macaca nemestrina) with risperidone, quetiapine, or placebo from 13-24 months of age, and will study a four-month post-drug period. Monkeys at one year of age are developmentally similar to children at 4 years of age in physical maturation, motor skills, physiology, and proportion reached in the species lifespan. Twelve males will receive a low dose of risperidone or quetiapine for 4 months, and then 8 in each drug group will be switched to a high dose for 4 months. Twenty-four males will be assigned to the placebo condition. Animals will be tested before, during, and after drug or placebo treatment for (1) social, emotional, exploratory, learning and memory, motor skill, and perceptual behavior; and (2) physical assessments of health, somatic growth, bone mineralization, immune system status, and hormonal function. The study design will permit both between-group and within-individual comparisons to examine drug group differences as well as alterations in individual growth rates caused by the drugs. Since pigtail monkeys demonstrate psychological and somatic development comparable to humans, this project will identify aspects of development in human children that are likely to be affected by chronic treatment with these agents.